Brake control mechanism



Nov. 15, 1938. w. F. BOLDT ET AL I BRAKE CONTROL MECHANISM Filed July 26, 1957 3 Sheets-Sheet l INVENTORS W. F. BO L D T BY gg.R." REE MAN ll FRONT BRAKES ATTORNEY.

NOV. 15, 1938. w, BQLDT ET AL, 2,136,410

BRAKE CONTROL MECHANISM Filed July 26, 1957 3 Sheets-Sheet 2 TRACTOR TRAILER INVENTORS W.F. BOLDT FREEMAN PatentedNov 15, 1938 Werner r. abut, Overland, and Walter a. Freeman, University City, Mo., assignors to Wagner Electric Corporation, St. Louis, M0,. a corporation oi Delaware Application July 26, 1937, semi No. 155,838

20 Claims. (Cl. 192-4) Our invention relates to improvements in means for preventing release of the brakes of a vehicle or vehicles from applied position under certain conditions.

One of the objects of our invention is to provide a braking mechanism for a vehicle or vehicles with means for holding certain of the wheel brakes of the braking mechanism applied under certain conditions and for holding other of the wheel brakes applied under other conditions.

A more specific object is to provide a braking mechanism for a vehicle or vehicles with means which, when the power clutch is disengaged, will prevent release of the rear wheel brakes on asingle vehicle or the wheel brakes on the rear vehicle of a plurality of vehicles when the vehicle or vehicles are facing downwardly on an inclined roadway and the change speed gearingis set in reverse, and also prevent release of the brakes on the front wheels of a single vehicle or the wheel brakes on the front vehicle of a plurality of vehicles when the vehicle or vehicles are facing upwardly on an inclined roadway and the change speed gearing is set in another Position.

Other objects'will become apparent from the following description taken in connection with the accompanying drawings in which Figure l is a schematic view of a portion of the fluid brake mechanism, the clutch mechanism,- and the change speed gearing of a vehicle, showing our invention associated therewith; Figure 2 is a longitudinal cross-sectional view. of the control valve mechanism for the brake mechanism; Figure'3 is a cross-sectional view taken on the line 3-3 of Figure 2; Figure 4 13 a schematic view similar to Figure 1 showingrour'invention e'mbodied in a fiuidbraking mechanism for a tractor-trailer vehicle; Figure 5 is a longitudial cross-sectional view of a modified controlva ve mechanism employed in Figure 4; and Figure 6 is a cross-sectional view taken on the line 6+8 of Figure 5.

Referring to Figures 1, 2, and 3, there is shown a hydraulic braking system comprising a brake pedal I connected by arod 2 to actuate the "piston 3 of a well-known master cylinder device 4. The fluid motors S for the front brake assemblies. 6 are connected to asu'pply conduit 1 and the fluid motors 8 for the rear brake assemblies a are connected to a supply conduit M. The single conduit ll leadingfrom the master cylinder is connected to the supply conduits l and i0 through our novel control valve me'cha nisni I! whereby fluid under pressure, when transmitted to fluid motors i and 8, may be maintained therein under certain conditions to thus hold the brakes in applied position. The control valve mechanism is shown in detail in Figures 2 and 3 and comprises a.casing I! having achamber i4 hi one end, a chamber IS in the other end, and-a central chamber ll. The chamber l4 is-connected with conduit 1 leading to the front brakes; the chamber I5 is connected with conduit i0 leading to the rear brakQS; and 10 chamber I1 is connected to conduit l'i leading from the master cylinder. The centralchamber il communicateswith chamber 14 by a passage ll andwith chamber" by a passage 19.

The passage 18 has threaded therein a tubular plugil-carrying a rubber valve seat 2| posi- I tioned in chamber and cooperating with this seat is a ball 22 free to roll on a track 23 (Figure 3) mounted in the chamber by means of, plug 20. I Similarly, passage II has threaded therein a tubulanplug 24 carrying a rubbervalve-seat 25 positioned in chamber -II and cooperating with this seat 'isa ball 20 free to roll on a track 21 mounted in the chamber by means of plug 24.

The central chamber II has-journaled therein 25 a cross shaft 28 carrying an arm 29 which proiects into a central slot 30. inc rod 8|. This rod is formed with fluted end po tions 32 and 33 which project into the tubular plugs 20 and 24, respectively. The grooves forming the fluted '30 portions are of such depth as to permit passage of fluid through the plugs and rod 8| is'of such over-all length that when arm 2| is in the ver-' tical position the ends of the rod will hold both balls 22 and 26 off their seats and permit fluid to 35 pass in either direction between the central chamber and the end chambers. Either ba'li may be permitted to seat by moving rod 3| in ,a direction to place the end adja ent the ball back of the'valve seat.- The'valve mechanism Just described is adapted to be controlled by both the clutch mechanism and the change speed gearing. Referring again to Figure l, the exterior end ofshaft It has secured thereto an arm '34 and mounted thereon are pivoted sleeves I and "t A rod it extends through sleeve 36 and is connected to the usual clutch pedal 3! aboveits' pivot point and a sec-, and rod 40 extends through sleeve 81 and is connected tothe clutch pedal below its pivot pointr The rod '38 is provided with a collar ll for eneasing the forward end of sleeve 36 and rod lll brake asemblies in Figure 1. Thus it is seen that by means of this construction when the clutch pedal is depressed to disengage the clutch, arm is is free to be swung in either direction but when the clutch pedal is in a position to permit the clutch to engage, the arm will always be brought back to its central position as shown in Figure l. I The clutch pedal is biased to clutch-engaged position against a stop fill by a spring (i l.

The lower end of arm is also provided with a pivoted sleeve t5 and extending through this sleeve is a rod it which is connected to the lower end of a lever Q3? pivoted to the gear shifting lever dome of the change speed gearing 55.. The upper end of lever ll is adapted to be positioned in the path of the gear shift lever 19 when this lever is moved. forwardly to place the in reverse speed as is the usual practice. llhe lever is held in its normal position against a stop by a spring 5i and when in this posi tion. it will not be affected. by the gear shift lever when it is moved to place the in forward speed gear position or in neutral pcsi tion. The rod is provided with coils..-

and E3 on opposite sides of sleeve and between these collars and the sleeve are springs and The collars are so positioned on th rod that spring will be compressed when arm is in its central position as determined. by the slut pedal when in clutch-engaging position. When the clutch is disengaged, springs Fi l and n-lll be come balanced and will be for wardly. When the gear shift lever is moved to reverse gear position, lever willie-e ved spring will become compressed. -Uz Le; these conditions if the clutch pedal is depressed, the springs will become balanced and swing arm to rearwardly.

Referring to the operation of the above described mechanism, the valve casing ii. is so mounted on the vehicle that the tracks 23 and ii are parallelwith the vehicle roadway, that is, horizontal when, the vehicle is on a level roadway. When the clutch pedalis in clutch-engag-= ing position and the gearing is in either neutral. position or any one of the forward gear positions, the parts will be in the positions shown in Figure 1. If the vehicle is stopped facing upwardly on an inclined roadway and the clutch is disengaged-arm 36 will be swung forwardly under the action of spring lid and, as a result thereof, rod 3! connected to the lever will be moved to the left as shown in Figure 2. -Due to the longitudinal inclination of the vehicle, ball 22 will now roll against seat 2!. If the brakes have been applied, they will be held applied on the front wheel only since ball 22 will prevent fiuid from returning tothe master cylinder. The rear brakes will be free to be released by the brake pedal as ball 28will roll tothe left end of chamber l5 due to the inclination of its track and also be prevented from seating due to the position of end 33 of the rod. If the brakes have not been applied, they all may be applied by manipulation of the brake pedal as the fluid under pres sure will unseat ball 22 and permit fluid to be transmitted to the front brake assemblies. It is thus seen that the front brakes only can be maintained applied without holding the operator's foot on the brake pedal when the clutch pedal is depressed, thereby leaving him free to use his right foot for other purposes; as for example, the starter pedal or the accelerator pedal preparatory to starting the vehicle. When the clutch pedal is released, arm 34 will again be brought to its center position as shown in Figure 1 and ball 22 will be forced ofi its seat by end 32 of rod 3!. The front brakes will then be released and the vehicle free to be started up the incline. The parts are so proportioned that the brakes will be released at the time or slightly before the clutch engages. From the foregoing it is apparent that the vehicle can be held by the front brakes from rolling backward on an incline when the gearing is in either neutral or forward gear position by merely holding the clutch pedal in clutch-disengaging position.

When the vehicle is stopped facing downwardly on an inclined roadway and it is desired to back the vehicle up the incline, the rear brakes can also be held applied without holding the operators foot on the bralre pedal, thus leaving him free to manipulate other pedals, as for example, the accelerator pedal. Under these conditions the gear shift lever is placed in reverse position and, as a result thereof, lever ll is moved in a clockwise direction. Th s causes spring to be compressed andspring to be released. With the clutch pedal depressed, spring 55 will swing arm 3 to the left and thus move rod ill to the right of its center position. Due to the inclination of the vehicle, ball will seatand ball will be either forced away or roll away from its seat. With the brakes applied the rear brakes will. be held applied due to the seating of ball The brakes will. be released by the release oi the clutch pedal in a manner already described.

By the use of the brake control mechanism just described the vehicle can be prevented from roling on an incline in a direction opposite to that for which the change speed gearing is set which is the desired condition as there is no difficulty in starting a vehicle which is free to roll on an inclined roadway in a direction corresponding to the gear setting.

It is also to be noted that by the use of the rolling balls to determine when the control valve mechanism will be effective to hold the brakes applied there will be no tendency of the brakes being held applied when the vehicle is moving.

Thus if the clutch is disengaged when the brakes are applied during the time the vehicle is moving forward along a highway, ball 22 will not become seated although the end 32 of the rod is withdrawn since the action of inertia during deceleration of the vehicle will hold the ball at the right end of chamber Hi. The end 33 of the rod under these conditions will hold ball 26 off its seat.

Referring to Figures l, 5, and 6, we have shown our invention embodied in a hydraulic braking system for a tractor-t ler vehicle. There is also shown in this embodiment a slightly modified form of control valve. The hydraulic braking system comprises a brake pedal 5%: connected by rod Ell to piston 58 of the master cylinder device 59. The tractor 60 is provided with brake assemblies 65, the shoes of which are actuated by the fluid motors 62, all of which are interconnected by suitable conduits connected to the supply conduit 63. The trailer M is also provided with wheel brake assemblies 65, the shoes of which are actuated by the fluid motor 66, all of which are interconnected by suitable conduits connected to the single supply conduit 51. A single conduit 68 leading from the master cylindot is connected to supply conduits 63 and 61 through the control valve mechanism 69 wherev by fluid under pressure, when transmitted to fluid motors 62 and 66, may be maintained there- 2,'1e,41o v .The lever I09 is held in its normal position in under certain conditions to thus hold the brakes of either the tractor or the trailer in applied position. r

Referring to Figures 5 and 6, the control valve mechanism comprises a casing 18 having a chamber II at one end, a chamber 12 at the other end, and a central chamber I9. The chamber II isconnected to conduit 63 leading to the tractor brakes; chamber I2 is connected to conduit 61 leadingto the trailer brakes; and the central chamber I3 is connected to conduit 68 leading from the master cylinder. The chamber 13 is connected with chamberII by a passageway 14 and with chamber I2 by a passageway I5. The chamber II has positioned therein a cage I8 formed of end plates H and I8 connected together by rods E9, the lower two of which (one only being shown) form a track upon which is positioned a ball 80. -The plate 'II carries a tubular projection 88 which projects through passage l t and into central chamber I3. The outer surface of this projection is provided with grooves 82 and the passage 83 in the tubular member is in constant communication with the central chamber I3 by holes 84. The plate TI and the tubular member carry a rubber member 85, one surface of which is adapted to engage the surface 86 surrounding the passageway M and these surfaces are normally biased in tight engagement by a spring 81 acting upon end plate E8 of the cage. The rubber member 85 also acts as a valve seat for the rolling ball 80 and-when this" ball is in engagement with'the rubber member, it will close off passageway 83 through the tubu-., lar member. I

The chamber [2 has positioned therein a cage 88 formed of end plates 89 and 90 connected together by rods 9|, the lower two of which (one only being shown) form a track upon which is positioned a ball 92. The plate 89 carries a tubular projection 93 which projects through passageway I5 and into the central chamber I3. The outer surface of this projection is provided with grooves 94, and passage 95 in the tubular membet is in constant communication with the central chamber 213 by holes 96. The plate 89 and the tubular member carry a rubber member 91, one surface of which is adapted 'to engage the surface 98 surrounding passageway l5 and these surfaces are normally biased in tight engagementby a spring 99 acting upon end plate 9001' the cage. The rubber member 9I also acts as a valve seat for rolling-ball 92Pand when this ball is in engagement with the rubber member, it will close off passageway 95 through the tubular member.

Referring to Figure 6, there is journaled in the casing I8 two aligned shafts I and IN, the inner ends of which project into the central chamber 73. The shaft I00 is.-provided with a rectangular-shaped cam I02 and shaft IN is provided with a triangular-shaped cam I03, both of which are adapted t'o cooperate with the ends of the tubular members M and 93. The outer end of shaft I00 carries an arm I04 which is connected by a link H05 to clutch pedal I0 8. The

exterior end of shaft Hill is also provided with an arm I01 connected by a link I08 with a lever I09 pivotally mounted upon the change speed gearing casing III! of the tractor. The upper end of lever I09 is adapted to lie in the path of movement of the gear shift lever III when it is moved forwardly from a neutral position to A place the change speed gearing in reverse gear.

against a stop II 2 by a spring II3.

Referring to the operation of the mechanism shown in Figures 4, 5, and 6, the valve casing I0 is mounted on the tractor so that rods I9 and rods 9I forming the tracks for balls 80 and 92 will be parallel with the vehicle roadway. When the: clutch pedal I86 is in clutch-engaged position (dotted lines in Figure 4), the-rectangular cam I02 will be in a position where its major axis is in substantial alignment with the axes of the cages, thus maintaining the valve cage I6 at.

' the right end of chamber II and valve cage 88 at the left end of chamber I2. When the gearing is in either neutral position or in any one of the forward gear positions, lever I09 will be in the position shown in Figure 4 and the triangular -cam I03 which is controlled by this lever will be in the position shown in Figure 5 wherein it will be capable of'maintaining cage 88 at the left end of chamber 12 as shown in Figure 5.

If the tractor-trailer vehicle is stopped facing upwardly on an inclined roadway and the clutch I is disengaged (shown in full lines in Figure4), arm IM will be swung forwardly to place the rectangular cam I02 in the position shown in Figure 5. The cage I8 will be free to be moved to the left end of chamber 'II by the action of spring 81 and the rubber member 85 will now seat against surfacev 86 andclose off any possibility of passage of fluid through grooves 82 in the tubular member 8|. Since the tractor is inclined upwardly, ball 90 will roll against the rubber member and also closeoff passage 83 through the tubula'rmember. If the change speed gearing is either in a forward gear position or in neutral position when the vehicle is stopped, cage 88 will be iii the position shown in Figure '5. The ball 92 will roll away from rubber member 91 and thus fluid will be free to flow to and fronf the tractor brakes by way of either grooves 94 or the central passage 95 in the tubular member. If the brakes have been applied by actuation of the brake pedal, they will be held applied on the tractor only since ball 80 will prevent return flow of'ffuid to the master cylinder from the fluid motors 62. If the brakes have not been applied,

tractor brakes only is sufllcient to prevent the tractor and trailer from rolling down the incline and since it is not necessary for the operator to hold his foot on the brake pedal, he is free to -employ it for other purposes, as for example,

operating the accelerator pedal for starting the vehicle or the starter pedal to start the engine in the event it has become dead. When it is desired to start the vehicle up the inclined roadly on an inclined roadway and it is desired to back the tractor and trailer up the incline, the trailer brakes can also be held applied without the operator holding his :foot on the brake pedal. Under these conditions the gear shift lever is placed in reverse gear position, thereby swinging lever iii?! rearwardly. This will cause the triangular cam Hill to become disengaged. from tubular member and engage the tubular member 3i to force cage it to the right where the rubber member 35 and ball Ilia cannot be effective to hold the tractor brakes applied under any conditions. "With the clutch disengaged, cage will be moved to the right end chamber 32 under the action of spring 33 and the rubber member ill will seal; against surface Due to the inclination of the vehicle, ball will now be in engagement with the rubber member and fluid will be prevented from flowing from the fluid motors oi the trailer back to the master cylincler. If the brakes have been applle", they will be held applied on the trailer only. If the brakes have not been'appliecl, they may be applied by actuating the brake pedal, ball unseating to permit fluid under pressure to be forced out of the master cylinder into the motors oi the trailer. When it is cleslrecl to start the vehicle up the inclined roadway, the clutch will be engage thus causing the rectangular cam to assume its horizontal position, thereby moving cage to the left and releasing the trailer brakes.

It is thus seen from the above description that when the valve control mechanism is employed on a tractor and trailer vehicle, the brakes on the tractor only can be held applied when the ve hlcle is stopped facing upwardly on an incline and that the brakes on the trailer only can be held applied when the vehicle is facing downwardly on an inclined rca ay.

Being aware of the possibility modifications in the particular structure herein described without departing from the fundamental principles of our invention, we do not intend that its scope be limited except set forth by the appended claims.

Having fully described our invention, what we claim as new and desire to secure by Let ers Pairent of the United $tates is:

1. In a vehicle, a oral-sing mechanism having a plurality braking units and means for normally controlling the application and release thereof, additional means for holding a braking unit in applied position when the vehicle is facing upwarden an inclined roadway and for holding another of the braking units in applied position when the vehicle is facing downward on an inclinecl roadway.

2. In a vehicle provided with a. plurality of vehicle braking units and means for normally controlling the application and release of the brakes, means other than said normal control means for holding a brake unit in applied position and other means for holding another-brake unit in applied position, and means under con trol of the vehicle operator determining which of said holding means shall be operative.

'3. In a vehicle equipped with front and rear brakes and means for normally controlling the application and release thereof, means for holol-.

ing the front brakes in apioliecl position under pro-selected conditions, and means for holding the rear b'rakesin applied position under other pro-selected conditions.

4. In a vehicle provided with a member for controlling the application of driving power to the vehicle wheels and with a plurality of vehicle aisaa'lo lei-clung units and means for normally controlling the application and release of the brakes,

means other than said normal control means for holding a braking unit in applied position and other means for holding another braking unit in applied "position, means under control of the vehicle operator determining which of said holding means shall be operative, and means for causing both of said holding means to be ineffective when the member is operated to cause driving power to be applied to the wheels of the vehicles.

5. In apparatus of. the class described, a braking mechanism having a plurality of braking units and means for normally controlling the application and thereof, a change speed gearing, and means other than the normal control means for holding certain of the braking units applied when the gearing is set in one position and for holding other of the braking units applied when the gearing is set in another position.

8. In apparatus of the class described, a b. l;-

having a plurality fora ng units and means for normally controlling the application and release thereof, a change speed gearing, means other than the normal control means for holding certain of the braking units applied when the gearing is set in one position and for holding other of the braking units ap= plied when the is set in another position, not manually operable means for causing both of I oltllng means to be ineffective. 7. l a motor vehicle, a braking mechanism havin a plurality of braking units and means for normally applying and releasing the same, means other than the normal means for holding certain of the bralzlngunits applied, additional means for holding other or" the braking units applied, manually operable means for selectively causing one or the other of said holding means to be operable, and other manually operable means for causing both of said holding means to be ineffective.

23. in a motor vehicle provided with a member o the vehicle wheels and with a braking mechvlng a plurality of braking units and Indy - braking units applied when the gearing is set in reverse position, additional means for holdlng other or" the braking units applied when the gearing is set in another position, and means for causing both of said holding means to be ineffective when the memberls operated to cause driving power to. be applied to the wheels of the vehicle.

9. In a motor vehicle provided with front wheel brakes and rear wheel brakes and means for applying the same and with a change speed gear ing, means for holding the rear wheel brakes applied when the gearing is set in reverse gear position, means for holding the front Wheel brakes applied when the gearing is set in another position, and means for causing both of said holding means to be lneflective at the will of the vehicle operator.

10. In a motor vehicle provided with a member for controlling the application of driving power to the vehicle wheels and with front wheel brakes the gearing is set in reverse gear position and the "or controlling the application of driving power tion and the vehicle is facing upwardly on an inclined roadway, and means for causing both of said holding means to be ineffective when the member is operated to cause driving power to be applied to the wheels of the vehicle.

11. In a motor vehicle provided with a change speed gearing and with two sets of brakes and means for applying the same, a gravity-controlled means associated with each set of brakes for holding the brakes thereof in applied position, ,and means controlled by the setting of the change speed gearing to cause the gravity-controlled means of one or the other of the sets of brakes to be efle'ctive to hold said set of brakes applied and prevent the vehicle from moving by its own weight on an'inclined roadway and in a direction opposite that for which the change speed gearing is set.

12. In a motor vehicle provided with a change,

speed gearing, with a clutch mechanism and with two sets of brakes and means for applying the same, a gravity-controlledmeans associated with each set of brakes for holding the brakes thereof in applied position, and means controlled by the setting of the change speed gearing to cause the gravity-controlled means of one or the other of the sets of brakes to be efiective to hold said set of brakes applied and prevent the vehicle from moving by its own weight on an inclined roadway and in a, direction opposite that for which the change speed gearing is set.

13. In a motor vehicle, two sets of fluid-actuated braking units, a source of fluid pressure, a conduit connecting each set ofbraking' units with the source of pressure, gravity and inertia controlled valve means associated with each conduit for preventing return flow of fluid from each set of braking units, a change speed gearing, means operable .when the change speed gearing is set in reverse gear position for causing one of said valve means to be operable when the vehicle is stopped and facing downwardly on an inclined roadway, means operable when the gearing is set in another position for causing the other valve means to be operable when the vehicle is stopped and facing upwardly on an inclined roadway, a clutch mechanism, and means operating when the clutch mechanismis in engaged position for causing said valve means to be ineffective.

14. In a. combined tractor and trailer vehicle, a set of brakes for thetractor, a set of brakes for the trailer, common means for applying the brakes on both the tractor and trailerjmeans for holding the tractor brakes applied, means for holding the trailer brakes applied, and means for determining, which of saidholding means is tobe effective.

'15. In a combined tractor and trailer vehicle, a clutch mechanism for the tractor, a set of brakes l for the tractor, a set of brakes for the trailer,

, trailer brakes applied, means for selectively causcominonmeans for applying the'brakes on both the tractor and the trailer, means 'for holding the tractor brakes applied, means-for holding the ingone or the other of said holdingmeans to be effective, and means operable when the clutch mechanismof the tractor is in engaged position v for causing both of said holding means to be inefiective.

16. In a combined tractor and trailer vehicle,

a clutch mechanism for the tractor, a change speed gearing for the tractor, a set of brakes for other of said balls brakes on the tractor applied when thetractor and trailer are stopped facing upwardly on an inclined roadway and the change speed gearing is set in another position, and means operable by the clutch mechanism when in clutch-engaged position for causing both of said holding means to be ineffective.

17. In a motor vehicle, two sets of fluidactuated braking units, a source of fluid pressure, a conduit connecting each set of braking units with the source of pressure, valve means in each conduit for preventing return flow of fluid from each set of braking units, each of said valve means comprising a valve seat and a rolling ball cooperating therewith and subject to the action of gravityand to the action of inertia during change in speed of the vehicle, a change speed gearing, a clutch mechanism, means operating when the clutch mechanism is in clutch-engaged position for causing both of said balls to be ineffective to prevent return flow of fluid, and means controlled by the gearing when in reverse speed and the clutch mechanism is disengaged for permitting one of the balls to be effective to prevent return flow of fluid when the vehicle is facing downwardly on an inclined roadway and when in another gear position and the clutch is disengaged for permitting the other of the ball to be eilective to prevent return flow of fluid when the vehicle,

is facing upwardly on an inclined roadway.

18. In a motor vehicle, two sets offluidactuated braking units, a source of fluid pressure, a conduit connecting each set of braking units with the source of pressure, valve means in each conduit for preventing return flow of fluid from each setof braking units, each of said valve means comprising a valve seat and a rolling ball cooperating therewith and subject to the action of gravity and to the action of inertia during change in speed of the vehicle, a change speed gearing, a clutch mechanism, a. movable member for selec tively permitting either of said balls to be effective to prevent return flow of fluid or both of said balls to be ineffective, means'operating when the clutch is in engaged, position for placing said member in a. position to cause both of said balls to be ineii'ective, and means operating when the clutch is in disengaged position and the gearing is in reverse for causing said movable member to assume a position where one of said balls is efiective to prevent return flowof fluid and when the clutchmechanism is in disengaged position and the gearing is in another position for causing said movable member to assume a position where the is efiective to prevent return flow of fluid. 3

19. In a motor vehicle, two sets of fluid-- actuated braking units, a source of fluid pressure, a conduit connecting each set of braking-units with the source of pressure, valve means in each conduit for preventing return flow of fluid from -me'chanism, a movable member for selectively permitting either of said balls to beefiective to the tractor, a set of brakes for the trailer, com- I prevent return flow of time or both ex" said "sails to be ineffective, e shaft for meving said member, an arm on said shaft, means contreiied by the gearing when in reverse gear for moving said arm to a position'where said meveeie memher Will permit one of said balls to be effective to prevent return flow of fluid and when in enether gear position for moving said arm to s ginsition whet said movable member "will ermit the ether et' essesao a track, a ball mounted on the track. em. subject to the action of gravity and inertia, :riwg change in speed of the vehicle and. en aYIDUIELJ. member carried by the end of t= e :e for cooperating with the valve seat and the M11, e chunge speed gearing, means comprising i; cam mem er controlled by the change speed gearing when in reverse gear for m ring one or the cages to a position where its annular member will be free of the valve seat and when in another gear position for moving the other cage to a, position Where its annular member will be free at the seat, 3. ciutchmechanism, and means ear uprising a cam for causing both of said cages to moved to pesition where said annular members are free of their vaive seats when the clutch is in engegeci eosition. 

